The invention relates to a cold-atom interferometry sensor.
The operating principle of such a cold-atom interferometry sensor using stimulated Raman transitions is in particular described in the application U.S. Pat. No. 5,274,232. These cold-atom interferometry sensors are known to have great sensitivity. In such a sensor, it is necessary to obtain two laser beams with different frequencies propagating in different directions in order to obtain atomic interference fringes from the emission of a cooled-atom source.
To obtain these two laser beams necessary for Raman pulses, it is possible either to use two distinct laser sources, for example contrapropagative, or to use a single source generating a first dual-frequency laser beam and a reflector arranged to reflect the laser beam so as to generate a second dual-frequency laser beam. This second type of sensor using a single laser source and a reflector for generating the two Raman pulse laser beams has the advantage of having good performance since the relative aberrations between the two Raman beams are reduced. Such an interferometry sensor using a reflector for generating the second Ramon dual-frequency beam is for example described in the application FR-A-2848296.
To improve the stability of the measurement supplied by the interferometry sensor, it is necessary to reduce the dispersion of an atomic source in terms of speed by cooling the atoms so as to obtain cold atoms. To do this, use is made of capture means arranged to capture the atoms issuing from the atom source so as to obtain cold atoms.
The invention relates more particularly to such a cold-atom interferometry sensor comprising:
a source of atoms;
a dual-frequency laser able to generate a first Raman dual-frequency laser beam;
a reflector arranged to reflect the first Ramon dual-frequency laser beam so as to generate a second Ramon dual-frequency laser beam, the first laser beam and the second laser beam propagating in different directions in order to obtain atomic interference fringes from an emission of cold atoms obtained from the source of atoms.
Such a cold-atom interferometry sensor is for example described in the doctoral thesis entitled “Characterisation of a cold-atom inertial sensor” by Florence YVER LEDUC, 2004, or in the publication “Six-Axis Inertial Sensor Using Cold-Atom Interferometry”, B. Canuel, F. Leduc, D. Holleville, A. Gauguet, J. Fils, A. Virdis, A. Clairon, N. Dimarcq, Ch. J. Borde, A. Landragin, and P. Bouyer, Phys. Rev. Lett. 97, 010402 (2006). In this document, and conventionally, the sensor comprises capture means arranged to capture the atoms issuing from the source of atoms so as to obtain cold atoms. As is also known, these capture means comprise a trap consisting of six lasers contrapropagating in the three directions in space. Such a cold-atom interferometry sensor therefore has the drawback of requiring at least one Raman laser for the atomic interference measurements, and several lasers for effecting the capture of atoms so as to obtain the cold atoms affording good interferometry measurement. As a result the cold-atom interferometry sensors of the prior art are complex and bulky.
The problem solved by the invention is providing one or more cold-atom interferometry sensors as described above requiring fewer lasers so as to be more compact, while enabling satisfactory measurements. According to the invention, this problem is solved by using the reflector no longer only for its function of generating the second Raman beam, but also for forming the capture means making it possible to obtain the cold atoms by means of multiple reflections of the first Raman beam on the surfaces of the reflector. More particularly, the problem mentioned above is solved by the fact that the reflector is also arranged to enable multiple reflections of the first beam on surfaces of the reflector so that the first beam and its multiple reflections make it possible to capture the atoms issuing from the atom source so as to obtain the cold atoms.
Thus, by virtue of the invention, the contrapropagating lasers forming, in the known devices, the capture means are no longer necessary since it is the first laser beam itself which, by means of multiple reflections on the reflector, provides the capture. Consequently the cold-atom interferometry sensor according to the invention requires only one laser source for performing both the interferometry measurements by Raman transition and the capture of the atoms in order to obtain cold atoms.
In the field of traps for obtaining cold atoms, the publication “Single-beam atom trap in a pyramidal and conical hollow mirror”, de Lee et al. Optics Letters August 1996 is known, which teaches that it is possible to trap and cool atoms by means of a reflector using only one laser. However, this publication does not concern the field of cold-atom interferometry sensors and in particular it is nowhere mentioned that the reflector forming a particular atom trap described in the publication can be used as a reflector for reflecting the Raman beam of a cold-atom interferometry sensor. On the other hand, according to the invention, it is indeed the same reflector that is used to effect the capture of atoms and the reflection of the Raman laser beam.
Advantageous embodiments of the invention are now described. Advantageous features of the reflector mentioned above are first described. This reflector can be arranged so that the first beam and the reflections of the first beam on the surfaces of the reflector constitute contrapropagating beam pairs for capturing the atoms so as to obtain the cold atoms. In this case, the reflector can be arranged so that the first beam and the reflections of the first beam on the surfaces of the reflector constitute three pairs of contrapropagating beams. This feature makes it possible to make a satisfactory capture of the atoms issuing from the atom source so as to obtain the cold atoms.
The reflector may be a convex reflector so that the first beam and the reflections of the first beam on the reflector make it possible to capture the atoms in the volume of the reflector. This feature of the reflector enables the reflections of the first beam to be directed towards the inside of the reflector so as to ensure a good capture. The reflector can in particular have a conical or frustoconical shape so that the first beam and the reflections of the first beam on the reflector make it possible to capture the atoms in the volume formed by the reflector. In particular, the reflector can have a pyramidal shape with a square or truncated pyramidal cross section so that the first beam and the reflections of the first beam on the surfaces of the reflector constitute three pairs of contrapropagating beams to capture the atoms in the volume formed by the reflector. This particular shape of the reflector then ensures good capture.
The reflector can be arranged so that the second laser beam propagates in a direction opposite to the direction of propagation of the first beam and preferably the reflector can be arranged so that the second beam has an identical polarization to the polarization of the first beam. This facilitates the obtaining of atomic interference fringes. To do this, the reflector may have a frustoconical or truncated pyramidal shape with a flat surface perpendicular to the direction of the first beam, the flat surface being treated so that the beam reflected on the flat surface has a polarization identical to the polarization of the first beam.
Other advantageous features of the sensor according to the invention are now described. The atom source may comprise an atom chip provided on one of the flat surfaces of the reflector in order to create an ultra-cold cloud magnetically trapped. This feature of the atom source improves the trapping of the atoms and cooling thereof.
The sensor may also comprise magnetic means arranged to trap the cold atoms magneto-optically, the magnetic means being arranged with respect to the reflector so that the cold atoms are trapped in the volume of the reflector. The magnetic means can also be arranged to generate a constant magnetic field so as to emit the cold atoms in order to obtain the atomic interference fringes. The emission of the cold atoms in order to obtain the atomic interference fringes is for example able to be carried out by gravity.
The atom source may be able to generate an atom vapor by at least one of the following methods:
desorption by heat,
light,
control of the temperature of a cold spot.
Such methods enable a satisfactory generation of atom vapour.
The sensor preferably comprises a vacuum chamber, the reflector being positioned in the vacuum chamber, and the sensor also comprising transmission means arranged to make the first laser beam enter the vacuum chamber. In this case, the transmission means may comprise a window transparent to the first laser beam.
The sensor may also comprise detection means arranged to detect the atomic interference fringes. These detection means comprise for example photodetection cells arranged to detect a resonance fluorescence emitted by the cold atoms.
The invention also relates to a system comprising a first interferometry sensor as described previously and a second interferometry sensor as described previously, the first sensor comprising a first Raman dualfrequency laser, the second sensor comprising a second Raman dual-frequency laser, the laser beam generated by the first laser of the first sensor having a propagation direction different from the propagation direction of the laser beam generated by the second laser of the second sensor, the system also comprising detection means positioned at the intersection of the propagation directions of the laser beam generated by the first laser and of the laser beam generated by the second laser.
This system may also comprise a third interferometry sensor as described previously, the third sensor comprising a third Raman dualfrequency laser, the laser beam generated by the third laser of the third sensor having a propagation direction different from the propagation direction of the laser beam generated by the second laser of the second sensor and from the direction of the laser beam generated by the second laser of the second sensor, the detection means being positioned at the intersection of the propagation directions of the beams generated by the first laser, the second laser and the third laser. In this way, it is possible to establish laser pulse sequences offering access to several inertial quantities successively, in particular in acceleration and in rotation.